This invention relates to heat conducting mechanisms; and more particularly, it relates to such mechanisms in integrated circuit packages, which conduct heat away from the integrated circuit chips that lie therein.
In the prior art, it is known that an integrated circuit chip dissipates heat while the chip is operating. Thus, the problem arises of how to prevent the temperature of the chip from exceeding a certain maximum level at which the chip begins to degrade in reliability or performance.
To solve the above problem, various mechanisms for conducting heat away from the integrated circuit chip have been disclosed. For general background mechanisms, see for example, U.S. Pat. No. 4,791,983 by E. Nicol and G. Adrian, entitled "Self Aligned Liquid-Cooling Assembly;" or see U.S. Pat. No. 4,879,629 by J. Tustaniwskyj, and K. Halkola entitled "Liquid Cooled Multi-chip Integrated Circuit Module Incorporating A Seamless Compliant Member For Leakproof Operation."
Now in each mechanism which conducts heat away from an integrated circuit chip, that heat will flow from the chip along a thermal conduction path to either the surrounding air or to a liquid coolant. Also, the thermal conduction path will include one or more joints between different components which may be soldered or pressed together.
In the case where all the joints are soldered, the task of taking the integrated circuit package apart in order to replace a defective chip is made difficult. On the other hand, in the case where one or more Joints are pressed, the thermal conductivity through them is reduced.
To address the above problem, two materials respectively known as a "thermal grease" and a "liquid metal paste" have been developed. These materials are placed in the joint to fill any voids which may lie therein; and they are described in U.S. Pat. No. 5,056,706 by T. Dolbear, C. Mackay, and R. Nelson entitled "Liquid Metal Paste For Thermal And Electrical Connections".
However, a drawback of the thermal grease is that it's thermal conductivity, in comparison to the thermal conductivity of a liquid metal, is relatively low. See U.S. Pat. No. 5,056,706 at column 2, lines 24-29.
Also, a drawback of the liquid metal paste is that for many paste compositions, the viscosity will be so low that the paste will not hold its shape. Consequently, a separate barrier must be provided in the integrated circuit package to prevent the paste from running. This barrier is shown in U.S. Pat. No. 5,056,706 as item 24 in FIG. 2 and item 26 in FIG. 3.
To increase the viscosity of the paste, the relative portions of the constituent materials can be changed. However, as the paste is made stiffer, the degree to which it fills the voids in the joint decreases; and thus the thermal conductivity through the joint decreases.
For example, FIG. 5 of U.S. Pat. No. 5,056,706 shows a phase diagram of a liquid metal paste which is a mixture of Al and Ga; and column 8, lines 44-47 says that "any mixtures of the Al and Ga between the lines 30 and 32 at the temperatures involved will remain a paste and be suitable for the applications herein discussed." However, a mixture at one extreme of 65% Ga and 35% Al is nearly a liquid (which will require a separate barrier to hold it in place); and a mixture of the other extreme of 1% Ga and 99% Al is essentially a solid (which is too stiff to fill voids or gaps in a Joint.
Further, even if liquid metal paste has an ideal viscosity, it still often requires a separate physical barrier to hold it in place. This occurs when the width of the gap which the paste is to fill varies significantly due to dimensional tolerances. In that case, a portion of the paste can get squeezed out of the gap and cause a short or other defect in the package.
Accordingly, a primary object of the invention is to provide an integrated circuit package in which the above problems are overcome.